The invention relates to joint packings, but more particularly, the invention relates to a packing with an elastomeric core that is radially extensible when externally clamped. Annularly shaped elastomeric core type packers are used extensively by the petroleum industry in a variety of tools that selectively seal various portions of an oil well casing. While separate tools may be required to perform various oil well operations, several of the tools use the same or similar type of sealing means in the form of an annularly shaped elastomeric core. The core is radially extensible when externally clamped along the longitudinal axis of the core. When clamped, circumferential portions of the core contact and press against an internal wall of a well casing to effect a seal. Differential pressure is customarily applied across the effected seal where pressure at the "up-hole" side of the joint packing in the well casing is higher than the pressures at the "down-hole" side of the packing. Of course, the direction of the pressure differential may be reversed in some applications. The differential pressure causes the packing to deform and distend along its longitudinal axis in the direction of the lower pressure. Present day packers have a high compression modulus (e.g., from about 525 psi to about 725 psi at 25 percent compression at 300.degree. F. on a first compression cycle) to resist such deformation while maintaining an effective seal. Some joint packings are made with two elastomeric stocks of different moduli where a softer elastomeric stock is located along a center portion of the joint packing for radial extensibility. Combinations of time, temperature, differential pressure, and compressive force affect the useful life of such packings as measured by their ability to maintain an effective seal when radially extended. Of these, temperature and differential pressure are, perhaps, most detrimental to an elastomeric core type joint packing. For example, a core type packing having approximately 4.5 in diameter, 3 in length, and approximately a 1 in thickness may maintain an effective seal at differential pressures from about 3500 psi to 4500 psi at 300.degree. F. The same type packing will only handle a differential pressure from about 1000 psi to about 2000 psi at a temperature of 500.degree. F. while maintaining an effective seal. Such packings will not seal a differential pressure from about 5000 psi to about 5500 psi at temperatures as low as 200.degree. F. A typical failure mode of such packings is a rupturing of the elastomer at the distended circumferential portion on a low pressure side with an associated loss of radial extension (and hence, effective sealing). Some petroleum well environments require sealing a casing at temperatures above 300.degree. F. and differential pressures greater than 4500 psi.